1. Field of the Invention
The present invention relates to the rapid depolymerization of polyhydroxy acids (PHAs) in water by heating under pressure. More specifically, the present invention relates to the recovery of hydroxy acid value from a polyhydroxy acid polymer-containing source such as food container trash.
2. Description of Related Art
Shaped articles of high molecular weight (at least 10,000, and normally 15,000 to 500,000 MW) polyhydroxy acids (PHA), particularly as polylactic acid (PLA, polylactide), and polyglycolic acid (PGA, polyglycolide), and copolymers thereof, have been known for years. An important property of these polymers is that they are slowly hydrolyzable and thereafter biodegradable to environmentally benign by-products. Consequently high molecular weight PHA polymer shaped articles are finding increasing application as replacements for polystyrene and other non-degradable polymers in products that will degrade in a landfill, such as fast food containers (Sinclair et al., WO90/01521, 22 Feb. 1990).
While this is a significant step in minimizing litter and long-term landfill disposal, discarding high molecular weight polyhydroxy acid articles for natural destruction by hydrolysis has the cost penalty of discarding the valuable polyhydroxy acid.
Although the hydrolysis of PHAs is well known, heretofore it has not been achievable in a time frame to permit recovery from other insoluble ingredients and reuse of the valuable hydroxy acid (HA) moities. In fact, although degradable, the time for degradation of high molecular weight PHAs is so long as not to offer a significant lessening burden on landfills.
Thus, there is a need for an economical method to recover and recycle the polyhydroxy acid content of this source of insoluble waste material and avoid burdening landfills with this waste.
The most economical routes for PHA production start with the acid such as lactic acid. The acid is converted to an ester, dimerized to a cyclic ring such as lactide, which is then polymerized to PHA. This is a complicated and costly process. See Bhatia U.S. Pat. No. 4,835,293 (May 30, 1989); Bellis U.S. Patent No. 4,727,163 (Feb. 23, 1988); Klootwijk U.S. Pat. No. 3,578,700; Hostettler et al. U.S. Pat. No 3,284,417; and De Vries U.S. Pat. No. 4,797,468 (Jan. 10, 1989).
Bhatia, U.S. Pat. No. 5,136,053 discloses the depolymerization of low molecular weight oligomers remaining after PHA polymerization. This patent application does not address the problem of recovery of the monomeric values from used high molecular weight PHA articles.
Copending and commonly assigned, U.S. patent applications Ser. Nos. 071,797,502; 071,797,503; 071,796,272; and 071,796,274 disclose the recovery of PHA's, respectively, in the presence of an alcohol and an acid catalyst; in water and acid; in the presence of specific amines, and in the presence of water and lower alkyl alcohol.
The aforementioned patents and patent applications are incorporated herein by reference.